The assignee of the present invention manufactures and deploys spacecraft having multiple antennas for provision of communications and broadcast services. RF autotracking (RFAT) is a known method for steering individual antennas mounted to a spacecraft platform (or other vehicle) to compensate for pointing disturbances experienced by the vehicle and the antenna. As illustrated in FIG. 1, a conventional RFAT subsystem 100, consists of an antenna 110 and a positioning mechanism (APM) 120, each mounted on the vehicle, a ground-based RF beacon 150, and a controller (RF Autotrack Processor) 140. Controller 140 generates and transmits actuation commands to APM 120 so as to cause the antenna to remain in a desired pointing orientation with respect to the ground-based RF beacon 150. The controller 140 normally operates based on error information sensed by RF sensor 130. A vehicle may have several RFAT subsytems, each having a respective antenna, APM, ground beacon, and RF sensor. Furthermore, a pointing orientation of antenna 110 may be adjusted by steering the main reflector 111, the splash plate 112, or both.
Each RFAT subsystem 100 on a given vehicle generally uses a dedicated, respective, ground-based RF beacon 150 that provides a ground reference from a fixed location. Controller 140 senses errors in the apparent position of the ground-based RF beacon 150 and corrects for the errors by sending commands to APM 120 to return the antenna 110 to its optimal pointing orientation. An error to be corrected may result, for example, from pointing transients experienced by the vehicle, or from antenna-related factors, such as thermally-induced distortion.
Loss of signal from ground-based RF beacon 150, whether due to scheduled maintenance or an unplanned outage, degrades the pointing performance of RFAT subsystem 100. To mitigate this problem, redundancy in ground-based RF beacon 150 may be provided, but this results in substantial additional cost.
As disclosed by Reckdahl, et al., in U.S. Pat. No. 6,720,918 (hereinafter, “Reckdahl”), assigned to the assignee of the present invention, absence of a ground beacon in an RFAT antenna system may be partially mitigated by provision of a thermal distortion estimation system. The estimation system models the periodic thermal distortion profile of an antenna, using a generally repetitive diurnal profile, adjusted by a more slowly time-varying seasonal profile. The system described in Reckdahl does not correct for pointing errors resulting from vehicle transient disturbances such as those resulting from maneuvers or from Earth sensor radiance gradient effects.